Abstract Multiple sclerosis (MS) is a debilitating neuroinflammatory disease of the central nervous system (CNS) with a broad range of neurological manifestations such as numbness, paralysis, and loss of vision. Disease pathology presents with massive perivascular lesions where inflammatory demyelination results in axonal damage, the main culprit for the loss of neuronal function in MS patients. Although a lot is known about cells and molecules involved with disease pathology, what cellular and molecular mechanisms initiate the immunological cascade against the CNS remain unknown. The earliest signs of lesions in both human MS patients and in animal models of MS are blood-brain barrier (BBB) disruption and activation of microglia, which are the resident immune cells of the CNS. Our previous in vivo imaging studies identified microglia as the earliest responders in experimental autoimmune encephalomyelitis (EAE, an established animal model for MS). We found that microglia cluster around vessels that leak blood factors into the CNS and thereby determine the perivascular locations where new lesions form. What causes early vascular alterations, local disruption of blood vessels, and recruitment of peripheral immune cells that form these perivascular lesions is not known. In this proposal, we will explore the cellular and molecular mechanisms involved with early vascular alterations and BBB disruption in EAE and MS. We will investigate whether early perivascular microglial accumulation in EAE involves signaling between microglia and the vessel wall, which causes such vascular alterations and drives immune cell recruitment to the CNS. Specifically, we will determine whether activated microglia communicate with the cellular constituents of the cerebral vasculature through the endothelin (ET) system, which is one of the main molecular mechanisms involved in the regulation of vascular tone, blood pressure, and blood flow. Besides altering vascular properties, ET-1 also has potent pro-inflammatory effects as it enhances trans- endothelial passage of monocytes and leukocytes. ET signaling has been implicated in cardiovascular diseases, such as hypertension and stroke, but little is known about its potential role in MS or its animal models. Our preliminary results and prior studies suggest that the ET system is a good candidate pathway for inducing reduced cerebral blood flow and vascular abnormalities in EAE and MS. Our proposed experimental approach combines pharmacological and genetic inhibition approaches with in vivo imaging of vascular disruption and microglia, macrophages, and T cells in mice undergoing the course of EAE. Moreover, we will seek to validate our preclinical animal model findings on ET pathway expression across different types of lesions from MS patients by using the unique human brain tissue bank that we have available at the Cleveland Clinic. Since ET receptor antagonists are FDA-approved for the treatment of hypertension, our studies have the potential to provide proof-of-principle validation that repurposing these drugs can be beneficial for the treatment of MS in human patients.